What Went Wrong While Rock Tumbling: Common Defects in Your Results and What To Do About It
There are bound to be some mistakes or bad batches of stones when you're first getting started rock tumbling. The major cause of unsatisfactory results usually stem from hastiness and lack of cleanliness. Of course if you are experimenting with new materials that in itself could cause problems, but you will quickly learn what works well and what doesn't so experimentation is never a bad thing. Try not to get discouraged if you're a beginner! The following common defects happen to both newbies and very experienced lapidaries, but can be prevented or solved by the suggestions below:
- Broken Stones. Breakage can occur in all stages of grinding and polishing. Stones mishandled in washing, internal fractures completing themselves in a later grind, too violent action inside the tumbler from high speed or low solution viscosity are common causes. To prevent broken stones, be more careful in choosing what goes in and inspect thoroughly for fractures.
- No polish. General haziness over the entire rock surface indicates that stones did not polish. This might be caused by accidentally using the pre-polish grit instead of the polish. It's also possible that carry-over abrasive from a previous step contaminated your batch. If a rock is pitted, teeny tiny grains of grit can stay in those pits and release in later steps when you don't want them to. Another thing to watch for is not to mix different types of stones together (soft and hard) in the same batch, and to be sure the tumbler barrel is as full as the Owner's Manual indicates it should be.
- Only Edges Polished. If the load is too far below half, you will need to use more material or filler to bring load up to proper level. Repeat pre-polish step again.
- Only Centers Polished. If slurry is too thin or there is not enough additives to carry the abrasive, then uneven polishing may result. If edge chipping is apparent, the batch much be returned to the coarse grind for 2 days, then retumbled through the subsequent stages using more thickener and additives. For minor edge chipping or more haziness at the edges, return batch to intermediate grind.
- Incomplete Polish. When stone is shiny but lackluster with a mediocre finish, you know it was not completely polished. Insufficient time in pre-polish or intermediate grind might be the reason this happened, or perhaps the barrel or stones were not thoroughly cleaned from the previous step. Incomplete barrel cleaning between stages or contamination grit on your hands can cause an incomplete polish. Repeat pre-polish step.
- No Polish Inside Indentations. If you don't have enough small stones in a batch, and stones are all the same size, the polish doesn't occur uniformly. Generally, only the harder stones will polish fully. To avoid going back to rough grind, add smaller pieces of already polished material to the batch in the prepolish stage.
- Flats. Flat spots can be caused by the stones sliding against the barrel wall instead of actually tumbling. Try increasing the speed and/or the number of chunks in load.
- Spalling. When pieces are knocked out of the center of the stone, usually in a half-moon shape, it usually indicates that the rotation was too rapid or that a load of all large stones was tumbled instead of using various shapes. This occurs mostly with vibratory tumblers. To solve the problem, return to rough grind until all gouges are gone. Complete subsequent steps with more additives. Slow speed of tumbler rotation if your model allows it, and if not, slightly increase the viscosity of solution. Adjust vibrating tumbler to a less severe setting.
- Blistering. When surface fractures or blisters occur it is usually on crystalline material such as amethyst. It is probably a result of too much impact between the stones because of low solution viscosity or not enough filler. The tumbling speed might also be to high. To remove blisters, return to rough grind. Sort and handle stones carefully after surface fractures have been ground out.
- Scratch Marks. Tumbling with too many preforms or slabs in the same batch can cause scratches. Instead, use a maximum of 20% slabs or preforms in the same batch. Tumble these with softer stone medium. Tumble only slabs with a maximum dimension of 30% of barrel diameter.